Object.observe: don't unnecessarily emit oldValue for reconfigurations of data properties

When a data property has its attributes changed but its value remains the same,
don't emit an oldValue. This makes the API more consistent by only emitting
oldValue when the value of a property has actually changed (or been removed,
in the case of a reconfiguration as an accessor property or a deletion).

Review URL: https://codereview.chromium.org/11820004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13565 ce2b1a6d-e550-0410-aec6-3dcde31c8c00